The GMP manager uses a copy of the update service's url formatting code and has
since fallen out of sync. We'll also want to use the same formatting code for
the system add-on update checks so this just exposes it in a shared API.
I've moved the contents of UpdateChannel.jsm to UpdateUtils.jsm and exposed
formatUpdateURL there as well as a few properties that the update service still
needs access to.
UpdateUtils.UpdateChannel is intended to be a lazy getter but isn't for now
since tests expect to be able to change the update channel at runtime.
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